The present invention pertains to a process for synthesizing in situ bimodal uniform emulsion polymer particles wherein the average diameter of the larger modal particles are approximately 1.1 to 6 times the diameter of the smaller particles. Each modal of polymer particles is substantially uniform in size. Thus, for example a latex can be produced comprising a distribution of uniform particles of about 2000 A intermixed with uniform particles of about 3000 A. The resultant bimodal polymer particles can be homogeneous or heterogeneous and are particularly useful for providing improved physical properties in coatings, plastics, and adhesives.
Although emulsion polymerization processes have been suggested in the past, such processes ordinarily require particle surface coverage of the surfactants must be less than 100% surface coverage or less than critical-micelle-concentration (CMC). However, too little surfactant or considerably less than 100% surface coverage of the polymer particles introduces stability problems and produces considerable excessive coagulation. In commonly assigned Ser. No. 837,390 filed Sept. 28, 1977, the second stage monomer addition can be carefully controlled wherein the second stage monomer adds partly to seed particles and partly generates new particles. The disclosed process for controlling the level of second generation new particles provides a stabilized interdispersion of heterogeneous composite particles by controlling the levels of total surfactant above 105% seed surface coverage as well as controlling the balance ratio of non-ionic surfactant to anionic surfactants.
It now has been found that bimodal latex particles can be produced by controlling the rate of monomer feed relative to the rate of monomer polymerization. In particular, the first stage polymerization comprises a monomer feed rate faster than the rate of polymerization whereby excess monomer always exists in the reaction mixture. The second stage polymerization process comprises a monomer feed rate less than the monomer polymerization rate whereby monomer reacts and copolymerizes without having excess unreacted monomer in the reaction mixture. The polymer particles thus produced are uniformly bimodal polymer particles wherein each modal is approximately monodispersed. The diameter of the larger modal of particles can be between 1.1 to 6 times the diameter of the smaller polymer particles. These and other advantages of the process will become more apparent by referring to the Drawings and Detailed Description .